This invention relates to the removal of hydrogen cyanide from gaseous process streams. More particularly, the invention relates to the treatment of such process streams with adsorbent beds containing zinc oxide to reduce the hydrogen cyanide content of such streams to very low levels of concentration.
Synthesis gas is an increasingly important feedstock in the chemical industry. Existing or proposed commercial processes using synthesis gas (i.e. gaseous mixtures containing hydrogen and carbon monoxide) include processes for the manufacture of methanol, ethanol, the production of aldehydes by the oxo process, the production of glycols using rhodium catalysts, and the production of purified hydrogen and carbon monoxide streams. In most of these processes, the use of sensitive catalyst materials requires that contaminants such as sulfur compounds and hydrogen cyanide be removed from the gas to concentration levels of less than 1 part per million, by volume (hereinafter referred to as "ppmv"), and often to levels below 0.1 ppmv.
Synthesis gas mixtures typically contain a variety of impurities among which are sulfur compounds such as hydrogen sulfide (H.sub.2 S), carbonyl sulfide (COS), sulfur dioxide (SO.sub.2) carbon disulfide (CS.sub.2) and methyl mercaptan (CH.sub.3 SH), as well as hydrogen cyanide (HCN), hydrogen chloride (HCl) and others. The relative concentrations of these impurities in the gas depends on the feedstock from which the synthesis gas is derived. Generally, a gaseous feedstock, such as methane, introduces less contaminants into the synthesis gas than liquid feedstocks, such as naptha, gas oil, atmospheric residue (the bottom fraction obtained from an atmospheric crude refining still) and vacuum residue (the bottom fraction obtained from the vacuum refining of heavy feedstocks such as crude oil and atmospheric residue). Coal derived synthesis gases generally contain the highest concentration of sulfur compounds.
Present purification schemes typically utilize a reactive liquid absorbent such as aqueous ethanolamines, alkali carbonates and hydroxides, or sodium thioarsenite as a primary purification agent to absorb high levels of the various species of impurities and generally reduce them to levels of about 1 to 10 ppmv. Alternatively, a non-reactive physical absorbent such as methanol at cryogenic temperatures may be used as the primary purification agent. Purifying the gaseous stream to a higher degree with such absorbents is uneconomical because of the disproportionately large amounts of energy which would be required to regenerate the spent absorbent.
Accordingly, the effluent gas from a primary purification step usually requires further treatment to reduce the impurities to acceptable levels. Adsorbents to accomplish such purification are extensively described in the prior art. The prior art literature relating to adsorbents for gaseous purification concerns itself, for the most part, with eliminating sulfur compounds from gas streams, in particular H.sub.2 S. Thus, for example, U.S. Pat. No. 3,441,370 describes the removal of H.sub.2 S with the use of a zinc oxide adsorbent at a temperature from ambient to 800.degree. F. The removal of COS and RSH is also suggested, but only at temperatures above 500.degree. F. However, no data is provided in the patent to support this suggestion. U.S. Pat. No. 4,009,009 describes the removal of COS from arsenic-free gas streams with the use of alumina-supported lead oxide. Great Britain Application No. 012,540, filed Mar. 29, 1976 (corresponding to German Offenlegungshrift 2,650,711 published June 10, 1977) discloses the use of zinc oxide as an absorbent for hydrogen sulfide. The examples of the application show the removal of carbonyl sulfide along with H.sub.2 S, but the presence of carbonyl sulfide in the inlet feed gas is said to be restricted to small amounts (page 4, col. 2).
U.S. Pat. No. 3,492,083 broadly describes the removal of H.sub.2 S and COS from an industrial gas using as an adsorbent a mixture comprising oxides of aluminum, zinc, iron and/or manganese in combination with oxides of the alkaline earth and/or alkali metals. Adsorption is carried out at a temperature of from 100.degree. to 300.degree. C. The examples of the patent only disclose the removal of H.sub.2 S and SO.sub.2 from the various gases. U.S. Pat. No. 4,128,619 discloses a desulfurization process carried out at a temperature from 100.degree.-400.degree. C. using zinc oxide as the adsorbent. Hydrogen sulfide is the only sulfur compound which is shown removed in the examples of the patent. U.S. Pat. No. 2,239,000 discloses the removal of sulfur from gas mixtures comprising hydrogen and carbon monoxide at a temperature from 400.degree. C.-600.degree. C. using catalytic mixtures of zinc and iron oxides or zinc and chromium oxides.
The removal of hydrogen cyanide from a gas stream using soda lime as an absorbent is disclosed in U.S. Pat. No. 2,423,689. The addition of up to 10 percent by weight of zinc oxide to the soda lime is suggested for purposes of increasing the life and hardness of the soda lime.
Thus, while zinc oxide is generally known in the prior art as an adsorbent for the removal of sulfur compounds, such as, H.sub.2 S and CH.sub.3 SH, there has heretofore been no appreciation regarding its capability as an adsorbent for HCN.